For many years the routine clinical procedure for determining the susceptibility of microorganisms to antibiotics has been a two-step operation requiring a minimum of 48 hours to complete. The first of these steps involves growing the microorganism from a sample and the second step involves subjecting the microorganism to various antibiotics in order to determine which antibiotic inhibits growth of the microorganism. During the time period required to conduct this susceptibility test it is entirely possible that the patient's condition will worsen or change drastically. Accordingly, there has been a recognized need for developing an improved system for determining the susceptibility of microorganisms to antibiotics within a shorter period of time. There has also been a desire for simplifying susceptibility testing and reducing the cost of such tests.
One approach for overcoming these problems is set forth in U.S. Pat. No. 3,957,583 where a cassette for conducting antibiotic susceptibility tests is suggested in which a culture medium is freeze-dried and applied to wells of the cassette. In order to conduct antibiotic susceptibility tests specimens suspected of containing a harmful microorganism are diluted in a predetermined quantity of water in a separate reservoir. The reservoir is then connected with the cassette by means of a needle inserted through a septum. Once the reservoir and cassette are connected a vacuum is drawn through the reservoir, thereby evacuating the interior of the cassette. Upon obtaining the desired vacuum, the upper end of the reservoir is vented to the atmosphere so that pressure on the diluent mixture of microorganism and water contained in the reservoir forces the mixture into the cassette. That diluent mixture flows through various filler passages and completely fills the wells in the cassette where it rehydrates culture medium contained in the wells. In order to fill the wells completely with the diluent mixture overflow cavities are connected to the wells for collecting air remaining in the passageways and wells. While the apparatus and process of the aforementioned patent can be used to significantly shorten the period of time required for making antimicrobial susceptibility determinations there are a number of significant drawbacks which exist with the apparatus and its utilization.
One of the significant drawbacks is the auxiliary equipment required for utilization of the cassettes. For example, a separate, relatively large reservoir interconnected with the cassette through a somewhat complicated needle-septum arrangement is required. In order to avoid any possible cross-contamination this separate reservoir must be replaced for each test. In addition, a relatively expensive vacuum chamber is required for obtaining a proper vacuum inside the cassette. Another disadvantage of the apparatus is the fact that it does not permit varying aliquot amounts of liquid to be used to rehydrate the wells. The cassette is designed in such a manner that the wells are filled up completely each time. Prevention of cross-contamination from well to well is dependent on the existance of inoculum in the feeder lines to the wells and the length of the feeder lines.